The invention relates to an apparatus for drawing and ironing metallic or metalloplastic cans.
During the forming of metallic or metalloplastic cans, by drawing and redrawing or drawing and ironing, two kinds of difficulties are encountered.
The first one is that Al alloys containing Mg (5000 series according to the Aluminum Association designations) have a tendency to to gall to the forming tools particularly to drawning and ironing rings (or dies). It is practically impossible to form cans with bare metal; the solution consists either to make specific surface treatments (see FR-A-2 627 189) or to coat with a plastic matter on the external or internal surface.
The first solution is relatively complex and expensive; the second one presents another difficulty which is the roll-back defect, which is more important when the height (H) over diameter (D) ratio is high and/or the thickness of the lateral wall (e) is low.
The defect is produced when the drawn receptable is extracted on the mandrel rod by the extractor. It takes the form of concertina folds in the body of the receptable, or rolling back of its free edge, or in the most serious cases of longitudinal tears in the body of the receptable and rolling of the intermediate strips into "petal" shapes. It results from a hooping effect or adhesion on the body of the receptacle to the mandrel rod during the drawing operation.
The fault is still more pronounced in cases where the drawn cans are coated internally with an organic material such as plastics; this has a spring back effect which accentuates the hooping effect.
Attempts have been made to remedy this defect in various ways, but the results obtained are still inadequate and/or uncertain:
by increasing lubrication at the can/mandrel rod interface by any known means. However, this solution has the following limitations: PA1 by using a mandrel rod made of a hard metal (tungsten carbide) instead of a steel one (modulus of elasticity 500 GPa instead of 200 GPa) ; PA1 by using a hollow deformable mandrel rod, under high hydraulic pressure during the drawing operation, and by removing the pressure before extracting the drawn can. PA1 by assisting extraction by the pressure of a fluid, preferably a liquid, through the mandrel rod; however the pressure is limited by the stability of the base and often proves to be inadequate for correct extraction. PA1 jo&lt;25 .mu.m or lo&lt;3 mm, the galling appears on bare 5000 Al alloys, and it is not possible to extract the metalloplastic drawn can. PA1 jo&gt;200 .mu.m, the external surfaces are dull, which must be avoided. PA1 lo&gt;50 .mu.m, the effectiveness is not improved, but also leads to more expensive tools, and essentially to larger dimensions which are not compatible with existing machines.
It does not allow one to obtain high rates of thickness reduction and hence limitation of the drawing ratio r (e.g. to a maximum value r=(E-e)/E.times.100=20 with e .ltoreq.0.15 mm for aluminium cans) and of the H/D value obtained.
As far as the die is concerned, improved lubrication gives a drawn external surface which is rough and dull in appearance rather than smooth and shiny, whereas the latter surface condition is most frequently required;
However this solution is mechanically complex, difficult to implement and expensive;